There are three common ways to stretch thermoplastic films. One is referred to as machine direction orientation (MDO) which involves stretching the film between two pairs of rollers. The film is pinched in the nip of a first pair of rollers, which are running at a relatively slow speed, and a second pair of rollers, downstream from the first pair, which are operating faster than the first pair. Because of the difference in run speeds, the film in between the roller pairs must either stretch or break to accommodate the difference. The ratio of the roller speeds will roughly determine the amount that the film is stretched. For example, if the first pair is running at 100 feet per minute (fpm) and the second pair is running at 300 fpm, the film will be stretched to roughly three times it original length. The MDO method stretches the film in the machine direction (MD) only. The MDO stretching method is used to create an oriented film or a microporous film, for example, where the film contains a dispersed inorganic filler which creates microporosity upon stretching. The micropores formed in the microporous film as stretched by MDO tend to be oval and are relatively large, for example, up to 1.5 μm microns on the long axis. An early example of stretching or orienting a microporous film to improve gas and moisture vapor transmission by differential speed rollers is U.S. Pat. No. 3,832,267 which issued in 1974. This '267 patent also discloses a second method of stretching which is called tentering. In simplest terms, the tentering method involves grabbing the sides of the film and stretching it sideways. For many years this was the only way to stretch film from side to side, or in the cross direction (CD). The tentering method tended to be slow and, because the forces are concentrated on the edges of the film, often the film did not stretch evenly. U.S. Pat. No. 4,704,238 discloses a tentering apparatus having a pre-heating zone and a stretching zone, followed by a heat setting zone to facilitate the stretching of a preformed blown or cast film. In this '238 patent, the thermoplastic film contains inorganic fillers such as calcium carbonate which, when stretched by tentering and/or heat tempering, produces a microporous film. Another example of MDO stretching of polymeric film and multilayered film is EP 848663 where an extruded film product may first be cooled, and then later heated and stretched to form a breathable film product. Additionally, the extruded film may be stretched immediately after extrusion, before it is cooled.
A third method of stretching involves incremental stretching of thermoplastic film. This method is described in the early patent literature, for example, U.S. Pat. Nos. 4,153,751; 4,116,892; 4,289,832 and 4,438,167. In the practice of this method, the film is run between grooved or toothed rollers. The grooves or teeth on the rollers intermesh without touching when the rollers are brought together and, as the film passes between the rollers, it is stretched. Incremental stretching has the advantage of causing the film to stretch in many small increments that are evenly spaced over the entire film. This results in a more evenly stretched film, something that is not always true for MDO stretching and is almost never true for tentering. Incremental stretching allows one to stretch the film in the MD, CD and at angle or any combination of these three directions. The depth at which the intermeshing teeth engage controls the degree of stretching. Often, this incremental method of stretching is simply referred to as CD, MD or CD/MD. A number of U.S. patents have issued for incrementally stretching thermoplastic films and laminates. An early example of the patent art which discloses a method of incrementally stretching film is U.S. Pat. No. 5,296,184. Other relevant patents regarding the incremental stretching of thermoplastic films and laminates include U.S. Pat. Nos. 6,265,045; 6,214,147; 6,013,151; 5,865,926; 5,861,074; 5,851,937; 5,422,172 and 5,382,461.
The above brief description of stretching techniques and apparatus to produce an oriented or stretched polymeric film illustrates the efforts that have been made to produce film products having desirable aesthetic and mechanical properties. Notwithstanding these efforts, there is an ongoing effort to improve known methods for producing thermoplastic films and laminates thereof to achieve quality products having improved properties. Moreover, there is a continuing effort to improve apparatus for producing oriented or stretched thermoplastic film without significant capital expenditures. It has been a very desirable objective to make improvements with savings in capital expenditures and processing costs, yet with production of quality products.